West Nile Virus (WNV) is an emerging mosquito-born flavivirus that can lead to fatal encephalitis in several vertebrate animal species. The elderly ar especially vulnerable to WNV infection with increased incidence of disease and death. Although it is well characterized that elderly humans often fail to mount protective humoral immune response after immunization with candidate antiviral vaccines, the mechanistic causes behind this are poorly understood. Indeed, little is known regarding the age-related changes which shape B cell-mediated memory. Like humans, aged-mice show increased susceptibility to severe disease after WNV infection. To understand the underlying mechanisms of these observations we will assess the B cell repertoire and function in adult (4-6 month of age) and old (>18 months) mice. We will compare the magnitude and quality of the memory development in adult and aged mice following WNV infection and the memory response following a secondary infection. We hypothesize that old mice will develop lower qualitative and quantitative B cell-mediated immune responses against WNV infections that manifests as reduced levels of neutralizing antibodies, fewer antibody secreting cells, and ultimately decreased memory responses following secondary infections. The results of this study should inform our understanding of the mechanisms leading to immunosenescence of the humoral response and frame the development of vaccines for the elderly against WNV and other globally relevant infectious diseases. PUBLIC HEALTH RELEVANCE: This study will advance our knowledge of the age-dependent decline in immune system effectiveness and help explain the age-related vulnerability to WNV infection. A better understanding of the causes and consequences of these processes may facilitate the generation of new strategies that improve vaccine responses, and thus limit the morbidity of infectious diseases.